Tall oil is an important co-product of the Kraft process for pulping wood, especially wood derived from pine trees. A resinous, oily liquid, tall oil comprises a mixture of rosin acids and fatty acids and may be used in soaps, emulsions, lubricants, fuels and other applications. Crude tall oil (CTO) usually contains rosins, unsaponifiable sterols, resin acids (such as abietic acid), fatty acids (such as palmitic, oleic and linoleic acids), fatty alcohols, other sterols, and other alkyl hydrocarbon derivatives. The fatty acid fraction of tall oil (TOFA or tall oil fatty acids) is used to produce soaps, lubricants, and other products. Other related products include TOFA esters and tall oil rosins.
Black liquor is the filtrate from a Kraft “pulp mill” that contains the chemicals washed from the pulp after the digester. The concentration of the black liquor varies according to the type of pulp produced and the efficiency of the washing step. It will typically be 12-20% dissolved solids as it enters the weak liquor system. The black liquor is concentrated as it is processed in the multi-effect evaporators to “strong” black liquor that will typically be 45-55% dissolved solids and then to 70-80% dissolved solids as it is processed through direct contact or indirect contact concentrator(s). The chemical composition of black liquor is quite variable and is mostly dependent on the type of wood pulped and the chemical composition of the liquor used to pulp the wood chips. The dissolved solids consist of both organic and inorganic fractions. The organic fraction varies according to wood and type of process but will typically be ˜80% of the dissolved solids while the inorganic fraction will typically be ˜20% of the dissolved solids. The organic fraction consists mostly of lignin and chemical compounds that form from the decomposition of cellulose such as hemicellulose, sugars, and organic acids. The organic fraction of the black liquor contains an extractives fraction which will contain 3-5% based on the total dissolved solids. It is in the fraction that the tall oil soap resides which will contain the fatty and resin acids that the tall oil fractionator is interested in.
Crude tall oil soap is normally separated from black liquor of the Kraft process and is sent to an “acidulation” unit in which the soap is acidified to convert the soap to crude tall oil. Acidulation generates a spent acid phase along with the crude tall oil. If that were the end of the story, phase separation might be academic. However, acidulation also generates precipitates, principally calcium sulfate, and a lignin phase or “rag layer” containing some crude tall oil, spent acid, and lignin. Clean separation of these four phases is essential for good economics, but it can be tricky to achieve. For another description of the four phases, see U.S. Pat. No. 4,238,304 (col. 2, II. 14-23).
Acidulation can be accomplished either batch wise or continuously. In a typical batch process, water, tall oil soap, and an acid (preferably sulfuric acid) are pumped into a large reactor, and the mixture is heated with steam and agitated until the mixture reaches 90-100° C. Thereafter, mixing is discontinued and the mixture is allowed to settle. Crude tall oil (upper phase) is first removed using a winch-operated skimmer arm or similar device. Spent acid is then removed. The remaining lignin phase can then be pumped out or treated with caustic in the reactor. In a “stacked” approach, the lignin phase is left in the reactor for multiple tall oil cooks. When enough lignin accumulates, it can be reacted with aqueous caustic to convert the precipitated lignin solids to soluble lignate salts. An advantage of the batch process is that with due care, relatively “clean” spent acid (i.e., spent acid that is relatively free of lignin) can be isolated.
Continuous acidulation processes utilize either gravity (with decanters) or centrifugal force (with centrifuges) to separate crude tall oil from a spent acid phase. Most of the lignin remains in the spent acid phase as it enters the decanter or centrifuge. Compared with the spent acid isolated from batch acidulation, the spent acid from continuous acidulations is relatively “dirty” because it contains dispersed lignin and a small but significant amount of crude tall oil. Our work here confirms that once it is separated from spent acid, this dispersed lignin phase can contain 5 wt. % or more of crude tall oil. Consequently, recovery of a substantial proportion of this “lost” crude tall oil represents a laudable economic goal. Unfortunately, unless the spent acid is returned to the plant's weak liquor system, continuous CTO plants usually allow only one chance to recover soap. Once the spent acid is separated from the products of the acidulation reaction, it is usually sent to a point downstream of the pulp mill's last soap recovery opportunity.
Dissolved air flotation (“DAF”), or more generally, dissolved gas flotation, has been widely used to clarify wastewater by removal of suspended matter such as oil or solids. Water (or wastewater) is saturated with air under pressure, and the air-saturated mixture is fed to a flotation tank or basin. As the pressure is released, tiny air bubbles float to the surface along with suspended matter that clings to the bubbles. A skimming device is used to remove the suspended matter from the liquid surface. Although dissolved air flotation has been utilized in the chemical industry, it apparently has not been used to treat spent acid from a crude tall oil soap acidulation process. DAF units are designed to operate at wastewater treatment temperatures below 40° C., while spent acid streams in a CTO plant are normally processed at more than 60° C.
In some cases, it would be desirable to combine a process for lignin removal with a crude tall oil soap washing process. Recently, we described a method for producing crude tall oil that utilizes an alkaline wash medium containing reduced amounts of lignates and inorganic solids to wash the crude tall oil soap (see U.S. Pat. No. 8,419,897). Part of the challenge in any soap washing scheme is to provide an aqueous wash stream that is reasonably free of lignin and other components that tend to emulsify or become entrained with the crude tall oil soap.
The industry would benefit from an improved process for recovering crude tall oil, particularly the crude tall oil that becomes entrained with lignin and is largely sacrificed in a spent acid phase following acidulation, particularly continuous acidulation, of crude tall oil soap. An ideal process would recover that crude tall oil while also providing a clarified acid phase that could be used, following treatment with caustic, as clean alkaline brine for soap washing.